The present invention relates generally to flyback power converters. More particularly, the present invention relates to an LED driver having a power factor correction stage with indirect current sensing circuitry that implements a low-cost, dimmable, class II single-stage flyback topology.
Light emitting diode (LED) lighting is growing in popularity due to decreasing costs and long life compared to incandescent lighting and fluorescent lighting. LED lighting can also be dimmed without impairing the useful life of the LED light source. Typical requirements for LED drivers include protective isolation between an unregulated DC power source and the regulated DC output voltage to the load, a need for constant current control, and a high power factor (i.e., a ratio for the real power flowing to the load with respect to an apparent power, ideally approaching 100% or in other words a value between 0 and 1).
“Flyback” converters are widely considered to be an optimal solution for LED driver circuitry because they can easily provide power factor correction, inherently create isolation between the power factor correction and load stages, and are of relatively low cost. However, due to the class II isolation, it is also difficult to sense the output current through the load for providing output current regulation.
With reference to an exemplary power conversion circuit 10 as is conventionally known in the art, as shown in FIG. 1, T1 is a flyback transformer that helps create the class II isolation. The load Rload is on the secondary side 13 of the transformer and is coupled to one ground GND_S, while the control circuit 12 is coupled on the primary side 11 of the transformer with another ground GND_P. The controller 12 in this example is a power factor correction IC with current control capability. An input terminal Ctr receives the current feedback control signal. Another terminal GD is an output for providing driving signals to the MOSFET Q1 gate drive.
To properly control the output current, current information has to be passed from the secondary side to the primary side of the isolation transformer. As shown in FIG. 1, a current sensing resistor R_I_sense senses the load current and a first OPAMP circuit 14 amplifies the sensing signal, which is then fed back to a second OPAMP circuit 16 to provide, e.g., PI (proportional and integral) or P (proportional only) control operations. The control output in this example is fed to the source/diode side of an opto-isolator 18 to form a current signal. The sensor/emitter side of the opto-isolator 18 will transfer the current signal from the diode side and feed it back to the input terminal Ctr for current regulation.
One of skill in the art may appreciate that such examples of traditional current control are not only lossy (because of the power dissipation in the current sensor R_I_sense and the OPAMP circuits 14, 16), but also relatively costly in nature.
Therefore, it would be desirable to provide accurate indirect sensing of the primary current for at least the objectives of high efficiency and relatively low cost.